All nuclei with odd number of protons or neutrons have an impulse moment or spin different from zero. The nuclei also have a positive electric charge which, together with the spin of the nucleus, produces for the nucleus a magnetic moment whose direction coincides with the spin axis of the nucleus. A field generated by the magnetic moment of a nucleus can be approximated by the field of a magnetic dipole. If a sample containing a large number of nuclei is placed in a static magnetic field, the magnetic moments of the nuclei tend to align parallel with and the sample will be provided with a net magnetization parallel to the external magnetic field. The net magnetization is proportional to the number of nuclei in a sample and to the strength of an external magnetic field (B.sub.o). The orientation of nuclei is disturbed by the thermal motion of nuclei, so the order of magnetization is also affected by the temperature of a sample. As temperature increases, magnetization decreases. In terms of quantum mechanics, the events can be described so that an external magnetic field generates a number of energy levels depending on the spin quantum number (I) of a nucleus, at which levels a nucleus can set with a certain probability. The nucleus of a hydrogen atom or proton has a spin quantum number I=1/2, so a proton can settle at two energy levels either in a manner that the direction of its magnetic moment is the same as that of the external magnetic field or reverse to this direction. Of these two, the former is more probable and the occupation proportions of energy levels conform with the so-called Boltzmann distribution.
In order to move from one energy level to another, a nucleus either receives or delivers an energy quantum as electromagnetic radiation of a certain frequency. The radiation frequency is determined by the difference between energy levels, which is directly proportional to the strength of the external magnetic field B.sub.o. This frequency, associated with exchange of energy, is called the Larmor frequency and this exchange of energy between nucleus and environment is called the nuclear magnetic resonance phenomenon. The principles of nuclear magnetic resonance have been dealt with in e.g. the following references: Abragam A.; The Principles of Nuclear Magnetism. London Oxford University Press., 1961 and Slichter C. P.; Principles of Magnetic Resonance, Berlin, Springer Verlag, 1981.
The nuclear magnetic resonance phenomenon has been studied by so-called continuous radiation (CW, Continuous Wave) and pulse methods. Pulse methods have been found more effective than CW-methods and are thus employed in NMR-spectroscopy and so-called nuclear spin imaging.
In pulse methods, a sample is subjected to an electromagnetic pulse of Larmor frequency, whose duration is determined in a manner that the nuclear magnetization of a sample tilts through a desired angle (.theta.) relative to the direction of an external magnetic field. The amplitude and duration of an electromagnetic pulse are generally selected in a manner that (.theta.) is a multiple of 90.degree. . Generally used terms are 90.degree. and 180.degree. pulses etc. The net magnetization Mn deflected from the direction of a basic magnetic field B.sub.o after the act of excitation precesses at the Larmor frequency W.sub.o around the direction of B.sub.o. This can be established by placing a coil outside a sample in a manner that its magnetic axis is orthogonal to the direction of B.sub.o. The precessing net magnetization induces in the coil a so-called FID-signal (Free Induction Decay), which oscillates at Larmor frequency and whose amplitude is proportional to the strength of the nuclear magnetization of the sample and to the strength of the external magnetic field B.sub.o.
The pulse methods associated with nuclear magnetic resonance tests have been described e.g. in the following references: Farrar T. C., Becker E. D.; Pulse and Fourier Transform NMR--Introduction to Theory and Methods. New York, Academic Press, 1971 and Ernst R. R., Anderson W. A.; Application of Fourier Spectroscopy to Magnetic Resonance, Rev Sci Instrum, Vol. 37, No 1, 1966.
During the excitation, a nuclear system receives external energy from an exciting radio-frequency field and, after the excitation, delivers it to its environment. The delivery of energy may occur as coherent radiation that can be detected by means of an external coil or the energy may be absorbed by the molecular structure of the sample. In connection with the delivery of energy, the net magnetization of a sample returns to its equilibrium value. The nature of this process is exponential and characterized by a relaxation time T.sub.1. This relaxation time is dependent on the composition of a substance to be examined, e.g. with liquid substances T.sub.1 is relatively short (milliseconds to seconds), while with solid substances, T.sub.1 is long (minutes to weeks).
The coherence of radiation emitted by a sample declines after excitation at a rate determined e.g. by the properties of a substance to be examined and by the homogeneity of the external magnetic field B.sub.o. This results in exponential decay of a signal at a rate characterized by a relaxation time T.sub.2 * (T.sub.2 asterisk or star). EQU 1/T.sub.2 *=1/T.sub.2 +.gamma..DELTA.B.sub./ (2.pi.), (1)
wherein
T.sub.2 is the spin-spin relaxation time of a sample PA1 .gamma. is a gyromagnetic ratio PA1 .DELTA.B.sub.o is the inhomogeneity of the polarizing magnetic field over a sample
The relaxation time T.sub.1.rho. is analogous to T.sub.1 longitudinal relaxation time and is the longitudinal relaxation time in a rotating frame of reference. This frame of reference rotates around the z' axis at the Larmor angular speed. The z' axis coincides to the z axis of the conventional frame of reference xyz. The main magnetic field B.sub.o is directed along the z' axis. The effective static field due to the magnetic field B.sub.o in the rotating frame of reference is zero. The magnetic component of the RF exciting field appears as a static field along the x' axis.
In the T.sub.1.rho. technique, the nuclear magnetization M.sub.o is tilted 90.degree. from the direction of the main magnetic field B.sub. o by a RF pulse. Thereafter, a RF field which has a magnetic component along the tilted magnetization is applied (locking field, B.sub.1). The magnetization of the spin system is now locked by the RF field. The magnetization relaxes towards the thermal equilibrium value which corresponds to the strength of the magnetic component of the locking field. Because the magnetic component of the locking field is much smaller than the polarizing magnetic field B.sub.o, this equilibrium value is approximately zero. T.sub.1.rho. is sensitive to low frequency processes and T.sub.1 is sensitive to processes which are at higher frequency. Because these processes are mainly due to the thermal movements, there exists a higher efficiency at lower frequencies than at higher frequencies and therefore T.sub.1.rho. .ltoreq.T.sub.1.
All the above relaxation times are dependent on the immediate environment of nuclei and its activity. As pointed out above, the physical state of a sample has an effect on relaxation times but also the strength of an external magnetic field and temperature of a sample change relaxation times.
The usefulness of the nucleus of a hydrogen atom or the proton in medical diagnostics is based on the abundance of hydrogen in soft tissues, in which it is primarily bound to water molecules. By virtue of its polarity, in turn, a water molecule links itself in various ways to different protein chains and this linkage is altered for a plurality of reasons, e.g. due to a pathological process.
Relaxation times and their variations have been dealt with in e.g. the following references:
R. Damadian US Pat. No. 3,789,832 and Nuclear Magnetic Resonance of Intact Biological Systems, Phil Trans R Soc Lond., 289, June 1980. R. Mathur de Vre; Progress in Biophysics and Molecular Biology, Vol. 35, 103-104, 1979.
Interest in utilizing the nuclear magnetic resonance phenomenon in medicine rose in the early 1970s. That was when R. Damadian published his research results, revealing that the relaxation time T.sub.1 of a malignant tumour tissue is even twice as long as that of a corresponding normal tissue. The publication R. Damadian U.S. Pat. No. 3,789,832 discloses a method for identification of a malignant tumour tissue by comparing the measured relaxation of a tissue with tabulated relaxation time values and then diagnosing possible malignancy of a sample.
However, later studies have indicated that the variation of relaxation times is not specific to any particular pathological condition. It can be generally concluded, however, that relaxation times change readily due to various ailments and can thus be applied in medical diagnostics.
The publication U.S. Pat. No. 3,789,832 also discloses a kind of scanning apparatus for the examination of a human body by means of NMR. However, this prior art solution cannot be regarded as any nuclear magnetic resonance imaging apparatus. The basic idea of nuclear magnetic resonance imaging was published by P. C. Lauterbur in 1973 in his reference P. C. Lauterbur; Nature, 242 190, 1973. In this publication, he also brought up the idea of mapping a relaxation time T.sub.1. Several pulse sequences have been developed for measuring relaxation times, including so-called Saturation Recovery and Inversion Recovery sequences for measuring T.sub.1 and Spin-Echo sequence for measuring T.sub.2. These sequences have been described e.g. in the reference: Farrar T. C., Becker E. D.; Pulse and Fourier Transform NMR--Introduction to Theory and Methods, Academic Press, New York, 1971.
The nuclear magnetic resonance imaging methods can be roughly classified in three categories: 1. Point scanning, 2. Line scanning and 3. Volume scanning methods. In point scanning techniques, an object area to be examined is mapped by moving the object or a point-shaped NMR sensitive area, obtained by various technical means, with respect to each other. The main disadvantage of single point techniques is that they are slow and, therefore, they are not applied in medical imaging. With special arrangements, however, the point scanning methods can be used to obtain more tissue information than e.g. with whole volume scanning methods. The single point scanning techniques have been described in the references: Tanaka et al: Proc. IEEE, Vol. 66, No. 11, 1582-1583, 1978, Damadian: Offenlegungschrift 2946847, Moore et al: U.S. Pat. Nos. 4,015,196, Abe: 3,932,805, Garroway et al: 4,021,726, Crooks et al: 4,318,043, Young: UK Patent Appln. GB No. 2122753 A.
By combining the slow single point scanning technique and rapid ultrasonic imaging technique, as set forth in the reference Sepponen; FI Pat. No. 64282, the single point scanning techniques can be utilized in medical diagnostics.
Line scanning techniques have been described e.g. in the following references: Moore et al: U.S. Pat. No. 4,015,196, Sepponen: FI Pat. No. 58868, Garroway et al: U.S. Pat. Nos. 4,021,196, Crooks et al: 4,318,043, Hutchinson et al: 4,290,019. The line scanning methods are also too slow for medical imaging and, thus, their application is restricted to certain special cases.
Imaging of a three-dimensional object is most preferably effected by applying whole volume scanning techniques. By means of so-called selective excitation, it is possible to define the object area to be examined and to effect more accurate mapping of the distribution of NMR parameters.
Selective excitation can be performed by exciting over the object a magnetic field gradient orthogonal to the plane of an object area to be excited and by modulating an exciting radio-frequency pulse in a manner that its frequency band width and the gradient field strength correspond to the width of a desired object area. Another method of defining an object area is to employ an oscillating magnetic field gradient, as in the reference: Moore et al: U.S. Pat. No.4,015,196. It is also known to utilize a gradient in an exciting radio-frequency pulse in a manner that on successive times of excitations the gradient direction is changed, a stable NMR signal being only generated in the plane where the pulse amplitude is constant.
A considerably more inaccurate method is to utilize the geometrical properties of transmitter and receiver coils for defining an object area and, thus, this method has only been applied when it is desired to make NMR spectroscopic studies of the object. The application of this method has been described in the references: Ackerman et al: Nature 283, 167, 1980, Haase et al: J. Magn. Reson. 56, 401-412, 1984, Bottomley et al: Radiology, 150, 441-446, 1984.
Whole volume imaging methods have been described e.g. in references: Lauterbur; Nature, 242, 190-191, 1973, Ernest; 4,070,611, Hutchison et al; International Patent Application No. WO 81/02788, Sepponen; FI Appln. No. 824343. For speeding up the imaging process, it is possible to apply methods described in references: Edelstein et al: GB Application No. 2079463, Mansfield; U.S. Pat. No. 4,165,479, Hinshaw; Physics Letters 48A, No. 2, June 3, 87-88, 1974, Likes; U.S. Pat. No. 4,307,343.
Particularly noteworthy nuclear spin imaging methods are so-called Fourier imaging methods, one version of which has been set out in the reference Ernst: U.S. Pat. No. 4,070,611. A drawback in this cited method is the collection of an FID signal generated after the excitation pulse. Encoded in the phase of a collected FID signal is the position information of one or two perpendicular directions by means of gradient pulses of constant amplitudes but varying durations. A drawback in this method is e.g. that the signal collection period changes on various signal collection events, resulting in the method's sensitivity to the inhomogeneities of the polarizing magnetic field B.sub.o. Also the T.sub.2 of a sample affects the signal to be collected.
The reference Hutchison et al: WO 81/02788 discloses a variation of Fourier imaging technique for generating a sort of spin echo by changing the direction of the magnetic field gradient. This spin echo is stored and in its initial phase is encoded the position information by means of a gradient pulse orthogonal to the direction of a read-out gradient, the amplitude of said gradient pulse being varied on various repetition cycles. A more preferred way of generating a spin echo is to utilize a so-called 180.degree. refocusing pulse which is capable of compensating the effect of the basic field inhomogeneities on a final image. Applications of this method have been described in references: Edelstein et al: EP 91008, Bottomley et al: EP 98426, Hutchison et al: Proceedings of 18th Ampere Congress, Nottingham, 1974, 283-284 and Sepponen: FI Appln. No. 824343 corresponding to U.S. Pat. No. 4,654,594 of March 31, 1987.
The reference Brunner P.: Journal of Mag. Res. 33, 83-106 (1979) discloses how to speed up the examination of a three-dimensional object with nuclear spin imaging techniques by directing the excitation and detection phases successively to various parts of an object. This serves to reduce the total examination time.
At present, nuclear magnetic resonance imaging is primarily used for mapping the hydrogen distribution of an object and the distributions of relaxation times T.sub.1 and T.sub.2 of the nucleus of a hydrogen atom. On the other hand, methods applicable for the mapping of relaxation time T.sub.1.rho. have not been published. An explanation to this may be that a strong radio-frequency pulse required in the methods for relaxation time T.sub.1.rho. measurements. The duration of the pulse is relatively long (several hundred milliseconds for biological tissues). At generally used operating frequencies of imaging equipment, this would lead to the absorption of radio-frequency power in the tissues of a person or test animal to be examined, which may result in local temperature rises and provide a health hazard.
However, nuclear spin imaging can be effected with relatively low field strengths (below 0.05T), the employed radio frequency being below 2 MHz and, as known in the art, at frequencies this low the radio-frequency electromagnetic power absorbance in body tissues is low.
One method of mapping the T.sub.1.rho. distribution of an object has been set out in the reference Sepponen FI Appln. No. 842292, corresponding U.S. patent application Ser. No. 06/738,850, filed May 29, 1985, now abandoned. However, measurement of the relaxation in the locking field can be used to obtain considerably more information than provided by merely mapping the numerical value of distribution of relaxation time T.sub.1.rho.. The reference R. R. Knispel et al: Dispersion of Proton Spin-lattice Relaxation in Tissues, J. Magn. Reson. Vol. 14, 44-51, 1974 has described the dependencies of relaxation times T.sub.1, T.sub.2 and T.sub.1.rho. on the strengths of a polarizing magnetic field B.sub.o and an irradiating locking field B.sub.1. Hereinbelow, the dependence of relaxation time T.sub.1.rho. on the strength of an irradiating locking field B.sub.1 will be called the B.sub.1 -dispersion of T.sub.1.rho.. An image of the local distribution of B.sub.1 -dispersion of T.sub.1.rho. formed of an object is called the B.sub.1 -dispersion map of T.sub.1.rho..
The relaxation rates of the proton resonance of body tissues are primarily affected by three mechanisms: The exchange of protons between water molecules, rotation and diffusion of water molecules. Thus, the total relaxation rate of speed is the sum of these three factors EQU R.sub.Total =Rex+bR.sub.rot +(1-b)R.sub.diff
Relaxation rates are dependent on the magnetic field strength in which relaxation occurs. Thus, R.sub.ex depends on the strength of an exciting magnetic field B.sub.1 and also on the strength of a polarizing magnetic field. R.sub.rot depends on the strength of a polarizing magnetic field.
R.sub.diff does not essentially depend on the strength of magnetic fields. These dependencies vary between different tissues and, thus, it is possible to characterize tissues by means of the dispersion of relaxation times. Relaxation time T.sub.2 does not essentially depend on the strength of a polarizing field but, instead, relaxation time T.sub.1 possesses a clear field dependency.